THE HYDROGEOCHEMICAL COMPARISON OF STREAMS AND LAKES IN FINLAND

A total number of 1165 stream water and 1172 lake water samples were c ollected in two different projects in the autumns of 1990 and 1987, re spectively. The sampling points in streams were determined to include a drainage area of ca 30 km(2). The lakes were selected to represent t he lakes in the size range of 0.01-10 km(2). The anions have different distributions in the stream and lake water data sets. Stream waters a re dominated by HCO3 (median value 200 mu eq/l), which comprises over half of the anion sum. Organic anions and sulphates are of the same or der of magnitude (ca 70 mu eq/l), respectively, followed by chloride ( 40 mu eq/l) and nitrate (ca 30 mu eq/l). In lakes, sulphate (85 mu eq/ l) and organic anion (76 mu eq/l) concentrations are higher than those of bicarbonate (64 mu eq/l). Chloride concentrations in lakes are com parable to those of the streams (28 mu eq/l), but nitrate (1 mu eq/l) and fluoride contents (<1 mu eq/l) are very minor. The median concentr ations of the main cations appear in the same successive order of magn itude in both stream and lake water data sets: Ca > Mg > Na > K > Al > H. The total amounts of cations (medians) in lake waters and streams are 260 and 450 mu eq/l, respectively. The hydrogeochemical maps compi led for this study show elevated concentrations in coastal areas for m ost of the elements particularly in stream waters, which have a more u niform sampling network grid all over the country. In many cases, the same areal pattern is discernible in lake waters, although there are o nly a few lakes along the coast as the selection was targeted mainly o n the upland areas with a higher lake density. These circumstances res ult in pronounced differences in the distribution of the concentration s. Small lakes show high individuality even in adjacent hydrological s ystems reflecting the different hydrographical conditions and soil and bedrock types of their catchments. The larger lakes with extensive ca tchment areas and higher amounts of dissolved solids have higher buffe r capacity than small lakes. Therefore, the chemistry of larger lakes is more compatible with that of streams.